In the past you needed large armies standing in a line, and tanks to crush that line. When armies made enough progress the remaining soldiers either surrender or ran away. Today you can have a handful people operating a drone to sink a warship, or fire a missile against a plane, or just planting random explosives and mines. They get killed and a new group pop up elsewhere doing the exact same thing. The only method to actually win by eliminating resistance becomes burning down every building and killing every person who used to live there, which obviously is immoral and hurts military moral, takes a lot of time, a lot of resources and people, and diplomatic resources. For a country like Ukraine or Russia it would take decades or centuries.